Question: In history class, the girl to boy ratio is $9$ to $3$. If there are a total of $36$ students, how many boys are there?
Solution: A ratio of $9$ girls to $3$ boys means that a set of $12$ students will have $9$ girls and $3$ boys. A class of $36$ students has $3$ sets of $12$ students. Because we know that there are $3$ boys in each set of $12$ students, the class must have $3$ groups of $3$ boys each. There is a total of $9$ boys in history class.